The present invention relates to a method and a device for switching from the inspiration phase to the expiration phase or vice versa during CPAP (continuous positive airways pressure) therapy in which a positive air pressure is continuously exerted on the airways (CPAP therapy).
The CPAP therapy serves for pneumatically supporting the larynx by continuously supplying a positive air pressure onto the airways. The pressure level is individually adjusted to the patient. The invention is used in a varied CPAP method wherein the expiration takes place at a lower pressure level than the inspiration. This has the advantage that the patient does not have to expire against the high pressure level. It is of great importance for the patient""s safety that the adjustment of the different pressure levels during the transition from the inspiration phase to the expiration phase and vice versa takes place with high precision.
In the state of the art, usually the respiratory gas flow from and to the patient is measured and differentiated with respect to time (first derivative) for obtaining a more distinct transition between the inspiration and expiration phases. The derivatives of the flanks of the gas flow curve are compared with threshold values indicating a transition to the respective other breathing phase. However, the patient""s respiratory gas flow may comprise deviations; furthermore, influences of the patient""s pulse-beat may have an effect on the gas flow. This may result in an overshooting of the first derivative wherein the threshold value for switching to the other breathing phase is achieved prior to the required time so that it is switched to the other pressure level and the patient""s safety may be jeopardized. However, it is also possible that the patient""s breathing is too shallow so that only a very low rise of the respiratory gas flow curve occurs in the inspiration phase. As a result, the value of the first derivative is so low that the threshold value for the transition to the inspiration phase is not reached.
A method for switching to the inspiration phase or expiration phase in the CPAP therapy is known from EP-A2-0 656 216. In this method, the motor speed and current consumption of a generator for supplying a patient with air is controlled and an operation signal is derived for the purpose of determining the inspiration and expiration phases. A signal representative of the gas flow to the patient is derived from the operation signal. The flow signal dxe2x80x9cflowxe2x80x9d/dt differentiated with respect to time is compared with a first and a second threshold value and the change in the breathing phase is derived therefrom.
It is the object of the invention to provide an improved method and an improved device for switching to the inspiration or expiration phase in the CPAP therapy, wherein the transition to the inspiration or expiration phase is detected with high precision and the patient""s safety is increased.
The object is achieved with the features of the claims.
According to the invention, the solution is based on the following principal concepts.
The first derivative of a respiratory gas flow curve from and to the patient is compared with two subsequent threshold values for the variation of the first derivative. The first threshold value is higher and therefore less sensitive. Deviations which often occur at the beginning of a breathing phase cannot reach said high threshold value and thus trigger any erroneous switching of a pressure level in the CPAP therapy. Subsequent to the high threshold value, a low threshold value which therefore is more sensitive is set for the purpose of determining the transition to the next breathing phase with high precision. The switch to the other breathing phase takes place when the first derivative has reached the second threshold value.
In the case of forced breathing, the high threshold may also be exceeded which is the reason why spontaneous breathing is possible at any time.
If the switching operation to the expiration phase explained above is not triggered, the switching occurs on account of a limiting time switching operation, e.g. at the latest after 3 to 4 s for the inspiration phase.
In a specific embodiment of the present invention, the time dependence of the gas flow in the expiration phase is sampled in a sequence of at least three sampling values in addition to the comparison with the second, low threshold value. If, in such a sequence, the condition is fulfilled that the second and the third sampling values exceed the respective preceding sampling value by a predetermined amount xcex94, i.e. the respiratory gas flow rises monotonically, the switch to the inspiration phase occurs at the third sampling value; if this switching does not occur, the switch to the inspiration phase is triggered by the comparison of the first derivative with the low threshold value.
In a preferred embodiment, the first derivative of the respiratory gas flow as well as the respiratory gas flow itself with the corresponding threshold values are monitored and switching to the inspiration phase occurs when one of the two parameters reaches the corresponding threshold value.
A further specific embodiment of the present invention relates to a method for detecting and excluding signals in the respiratory gas flow curve which originate from the pulse-beat (influence of the cardiac activity) of the patient. As a criterion for a pulse-beat signal, the time between its maximum and its minimum and the difference between the maximal and minimal gas flow is used. Upon detection of a pulse-beat signal, the monitoring of the monotonic rise of the respiratory gas flow is stopped or set to a less sensitive value for a short period of time (preferably 1.5 s). Additionally or alternatively, the threshold values for the comparison of the first derivative of the respiratory gas flow can be changed, preferably set to less sensitive values, upon detection of such a pulse-beat signal.
The advantages of the present invention reside both in a lower susceptibility to trouble and in a higher precision during switching to the inspiration or expiration phase in the CPAP therapy.